[GitHub] [couchdb] kocolosk opened a new issue #2227: Skip reading the revision meta when requesting a specific revision

[GitBox <gi...@apache.org>]

kocolosk opened a new issue #2227: Skip reading the revision meta when requesting a specific revision
URL: https://github.com/apache/couchdb/issues/2227
 
 
   ## Summary
   
   This one is a less frequent request path but I thought I'd log it anyway. Typically when a user requests a document, we execute two reads in FDB: one read to get the revision identifier of the current winning revision, and a second to get the body of that revision.
   
   When a user requests a specific revision I think we can omit the first read and jump straight to the `?DB_DOCS` space. But we don't. Here's a trace of me requesting a specific revision of a document; the first read GetRange call still goes to the revisions subspace:
   
   ![open-doc-revs](https://user-images.githubusercontent.com/16679/66129051-242cb480-e5bd-11e9-82cc-54755b705b88.png)
   
   (the two orange elements are checking metadata versions; I've filed separate issues to address them).
   
   ## Desired Behaviour
   
   When a user requests a specific revision on its own we should be able to retrieve that with a single GetRange request.
   
   ## Possible Solution
   
   I believe the current code for retrieving specific revisions is here:
   
   https://github.com/apache/couchdb/blob/ff2cdb84f639ea26ac7b1ad85abb0a49c5141c74/src/fabric/src/fabric2_db.erl#L513-L550
   
   There's a lot of extra work, retrieving *all* revisions of a document and building a `couch_key_tree` in memory in order to compute the revision identifier, before finally requesting the revision body at L540:
   
   https://github.com/apache/couchdb/blob/ff2cdb84f639ea26ac7b1ad85abb0a49c5141c74/src/fabric/src/fabric2_db.erl#L540
   
   The fix here would involve some streamlining of that function. There's a decent amount of subtlety there with options like `latest=true`.
   
   ## Additional context
   
   This optimization is probably mostly useful for the replicator, which does make requests for specific document revisions all the time.

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